Suppression of mTOR Signaling Attenuates Apoptotic Effects Induced By the NEDD8-Activating Enzyme Inhibitor Pevonedistat (MLN4924) in Acute Myeloid Leukemia Cells

Acute myelogenous leukemia (AML) is a heterogeneous disease and often relapses after standard chemotherapy. Therefore, novel therapies for AML are needed. Recently, the NEDD8 conjugation pathway and the mammalian target of rapamycin (mTOR) signaling pathway have emerged as important regulatory pathways for cancer therapy. Since some components of the mTORC1/2 pathways such as Deptor are neddylation targets, we postulated that combined exposure to NEDD8-activating enzyme (NAE) inhibitor pevonedistat and the dual mTORC1/2 inhibitor sapanisertib (TAK-228) might have additive or synergistic effects in AML.

To test pevonedistat and sapanisertib effects on AML cells, we used the agents alone or in combination followed by assessment of proliferation, apoptosis and signal transduction in human AML cell lines as well as in primary cells. By AnnexinV/7AAD flow cytometry analysis, we found that pevonedistat significantly induced AML cell death and apoptosis in a time- and dose-dependent manner. Western blot showed that poly (ADP-ribose) polymerase (PARP) and caspases 3/8/9 were cleaved, indicating that pevonedistat-treated cells were actively undergoing apoptosis. Among the six AML cell lines (HL-60, KG1a, Molm-13, MV4-11, THP-1 and U937) tested, MV4-11 was the most sensitive cell line (IC50 <25nM) and KG1a was the least sensitive cell line (IC50 >1000nM) after 48 hours of exposure. The average IC50 on primary AML blast cells (5 primary samples) was 400nM at 48 hours. Western blot analysis showed that the neddylated cullin levels were markedly decreased confirming inhibition of the NEDD8 conjugation pathway by pevonedistat. We also found that pevonedistat induced morphologic changes and increased CD11b expression in Molm-13 and MV4-11 cells, indicating cell differentiation.

The effects of pevonedistat on AML cells could be attenuated by pre- or co-treatment with the mTORC1/2 inhibitor sapanisertib. We found that sapanisertib alone induced limited cell death in most AML cell lines and primary AML patient blasts. The results from XTT assay and flow cytometry analysis indicated that sapanisertib alone significantly decreased cell size, arrested cells in G0-G1 phase and reduced protein content of AML cells but did not induce robust apoptosis. This suggested that through inactivating mTOR signal pathway, sapanisertib suppressed cell activity and induced a dormant/quiescent status and thereby attenuated the apoptotic effects of pevonedistat. With concurrent pevonedistat and sapanisertib exposure, the expression of apoptotic mediators such as cleaved PARP and caspases 3 were decreased compare to pevonedistat treatment alone (See Figure).

To confirm the role of mTOR inhibition on pevonedistat-induced AML apoptosis, we targeted mTORC1/2 by knocking down mTOR expression with small hairpin RNAs (shRNA). Downregulation of mTOR expression mirrored the effects of sapanisertib. mTOR inhibition by shRNAs resulted in decreased AML cell size and arrest of cells in G0-G1 phase. In addition, pevonedistat-induced cell death was significantly impeded in the cells infected with mTOR shRNA lentivirus compared to scrambled shRNA (p<0.01). Therefore, we validated that mTOR signal inactivation protects AML cells from NAE inhibitor pevonedistat-induced apoptosis using both pharmaceutical and genetic approaches. To further study the mechanism behind mTOR inhibition inducing resistance to pevonedistat, we tested rapamycin, an mTORC1 inhibitor. Concurrent exposure to rapamycin did not significantly alter the effects of pevonedistat and did not change cell size and cell cycle status. In addition, rapamycin did not inhibit the phosphorylation of 4E-BP1, suggesting that mTORC1 inhibition alone may be insufficient to inhibit the effects of pevonedistat, and suggesting a role for mTORC2.

Taken together, our data demonstrate that the NEDDylation pathway inhibitor pevonedistat has strong potential as a therapeutic agent for AML therapy via apoptosis induction. However, when AML cells are arrested in G0-G1 with reduced proliferation rates by dual mTORC1/2-inhibition, the ability of pevonedistat to induce apoptosis is suppressed. Whether this is due solely to the cytostatic effects of dual mTOR inhibitors or also involves other mechanisms remains uncertain. While both sapanisertib and pevonedistat have clear anti-leukemia effects, their concurrent use may inhibit NAE inhibitor-induced apoptosis.

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